There exists at least two relevant trends in the voice telephone terminal equipment industry which have given rise to the need for my invention: the first is reflected in the efforts of private automatic branch exchange (PABX) designers and manufacturers to merge key system functions into more sophisticated combinations of PABX common control equipment and associated telephone stations. The second is reflected in the efforts of key telephone system (KTS) designers and manufacturers to merge certain PABX features, including reduced requirements for station cabling, into more efficient electronic key telephone systems. This is illustrated, for example, by the electronic key telephone system shown in U.S. Pat. Nos. 3,637,939, issued Jan. 25, 1972; 3,843,845, issued Oct. 22, 1974; and 3,973,085, issued Aug. 3, 1976.
In both instances, and as one means of achieving at least some of their respective technical and functional objectives, manufacturers and designers have redistributed many of the traditional control functions from either the PABX or KTS common equipment to the station, and have otherwise employed sophisticated station electronics. One characteristic feature of the convergence of key system and PBX technology is reflected in the near elimination of the normally required multi-pair cabling system traditionally used with key systems or key system/PBX combinations, and now being replaced by so called "skinny wire" systems, or systems which use from three to as few as one pair of wires between the station telephone and the electronic PABX or key telephone system common equipment.
As a result of reducing the amount of cable connecting the stations to the common equipment, there now exists fewer cable pairs, or leads, over which to transmit the speech communications and the many forms of control signalling which normally operate between the station and common equipment. Thus, leads which were formerly dedicated to certain control functions--such as the A A1 L and LG leads of conventional 1A2 key telephone systems--have now been eliminated in the interest of reducing the amount of cabling required; meanwhile greater use of station electronics is serving to achieve or even expand the level of station control signalling capability.
New electronic stations typically use a separate data pair to the common control equipment to transmit station control signals in the form of either electronic pulses, or frequency tones. Speech signals and network control signalling, including ringing signals from the common control to the station, are transmitted over a separate, dedicated talking pair in the manner of conventional PBX systems. In such systems, however, problems arise in connection with efforts to return station status or other station control signals to the common equipment in a manner which will not interfere with the station's ordinary ability to receive ringing signals, and without otherwise exposing the station control electronics to potentially hazardous conditions which might occasionally be transmitted inadvertently over the ringing signal path. For example, and depending on how much control signalling is to be originated in the station and transmitted to the switching equipment, the control signalling circuitry at the station end of the loop must be able to generate a variety of discernable control signals without blocking or distorting ringing signals to be received by the station network. The control signals generated from the station are usually low voltage current pulses of various duration, generated by manually interrupting the normally present loop current, or "talk battery"; the ringing signals are low frequency signals of relatively high voltage, usually in the range of 80 to 120 volts AC.
Where a single pair of wires is to be shared for the transmission of all station control signals, and where the various control signals consist merely of loop current impulses of varying duration, faulty station control signals frequently occur due to the inability of the switching equipment to discern or identify the signal being transmitted from the station. Also, where the pulse signal is manually mechanically initiated, an erroneous control signal may be generated by (1) improper operation of the manual signal initiating device by the user, or (2) a faulty mechanical device. In such instances the station control signal pulse may be misinterpreted by the switching equipment, resulting in an improper line disconnection or other unintended operation on the line.
Among the kinds of station control signals transmitted over two-wire station loops associated with electronic switching equipment and stations are the line disconnect and line hold signal functions. These signals are typically initiated at the station by depressing the hook switch to disconnect the line, and by depressing a hold button (or, alternatively, the hook switch again in the case of some systems) to signal that a presently connected line should be put on hold, and the talk path made available for other use. Where loop current pulses of varying width or duration are used for the control signal, it is imperative that the pulses be generated accurately and uniformly in order that they might be interpreted properly by the switching equipment. Otherwise, one signal may be misinterpreted for another, or a received signal may not be recognized at all. In fact, it is frequently the case that users manually depress the hold button (or hook switch) longer than necessary, and thereby generate a loop current pulse of excess duration which is misinterpreted as a line disconnect signal. On the other hand, users frequently mislay the station hand-set when intending to return it to the hook switch at the completion of a call, such that the hand-set "bounces" off the hook switch, thereby sending a momentary pulse to the switching equipment which, in turn, would be misinterpreted merely as a "hold" signal, rather than a line disconnect signal.
The ability of the switching equipment to recognize and thereby respond to the station control signal is further impeded by long station loops--particularly in the case of off-premises extension (OPX) loops. Longer loops expose the control signals to transients, and increase the amount of pulse spreading. Either or both of these conditions can cause the original pulse characteristics to be lost, particularly in the case of manually or mechanically generated pulses of marginal original quality. Pulse distortion interferes with the ability of the switching equipment to recognize or discriminate between various control signals.